Sensors and Actuators B 201(2014)114–121
Contents lists available at ScienceDirect
Sensors and Actuators B:
Chemical
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s n
b
Ammonia gas sensors based on ZnO/SiO 2bi-layer nano?lms on ST-cut quartz surface acoustic wave devices
Yong-Liang Tang a ,Zhi-Jie Li a ,Jin-Yi Ma b ,Yuan-Jun Guo a ,Yong-Qing Fu c ,Xiao-Tao Zu a ,?
a
Department of Applied Physics,University of Electronic Science and Technology of China,Chengdu 610054,PR China b
Sichuan Institute of Piezoelectric and Acousto-optic Technology,Chongqing 400060,PR China c
Thin Film Centre,Scottish Universities Physics Alliance (SUPA),University of the West of Scotland,Paisley PA12BE,UK
a r t i c l e
i n f o
Article history:
Received 7February 2014
Received in revised form 2April 2014Accepted 12April 2014
Available online 2May 2014
Keywords:SAW sensor Ammonia gas ZnO SiO 2
Bi-layer
a b s t r a c t
Surface acoustic wave (SAW)ammonia gas sensors based on ZnO/SiO 2bi-layer nano?lms on ST-cut quartz surface acoustic wave devices were fabricated and characterized.The ZnO and SiO 2layers were coated onto SAW resonators by combining a sol–gel process and a spin-coating technique.The SEM and AFM results revealed the ZnO/SiO 2bi-layer ?lms had porous structures.The gas sensing results showed that the sensitivity of sensors was dependent on the value of sheet conductivity of the sensing ?lms.As a result,the bi-layer nano?lms were much more sensitive than the single layer ?lms due to their appropriate sheet conductivity,and the absolute response value was dependent on the thickness of the top ZnO layer.The sensor based on the bi-layer nano?lm with 60nm top ZnO layer showed the best gas sensing property.It exhibited a frequency shift of 2000Hz in 30ppm ammonia gas with good repeatability and stability.
?2014Elsevier B.V.All rights reserved.
1.Introduction
High performance and low cost gas sensors are required for many industry applications,such as environment pollution detection,toxic gas leakage,detection of explosives,combustible,?ammable and toxic gases,oxygen depletion,detection and warning systems for domestic and military bases,battle?elds,petrochemical processing,mining,tunneling and offshore indus-tries,automotive industry,petrochemical industry,etc.[1–4].Ammonia is one of the most dangerous industrial gases,and it is mainly generated in manufacturing of nitrogenous fertilizers,industrial refrigerant,and manure from agriculture and wildlife.It could cause poisoning and harming of people as well as explosion once there is a leakage of ammonia even in a low concentra-tion level,such as ppm or even ppb [5].Among different types of ammonia sensors available,such as electrochemical sensors [6],semiconductor sensors [5,7–10],surface acoustic wave (SAW)sen-sors [11–13]have their advantages of high sensitivity,high speed,good reliability,high accuracy and low cost.The high sensitivity associated with the SAW sensor results from the fact that most of the wave energy is concentrated on the SAW device surface within one or two wavelengths,therefore,any surface perturbations in the
?Corresponding author.Tel.:+86136********.E-mail address:xtzu@https://www.doczj.com/doc/6f7412771.html, (X.-T.Zu).
physical or chemical properties on the SAW device surface,such as mass loading,conductivity,temperature,and pressure,etc.,will have a signi?cant impact on the propagating waves.A SAW gas sensor can be based on the change of conductivity [14].A typical SAW gas sensor is made up of an oscillator with a SAW resonator [15],which is based on a piezoelectric substrate on which inter dig-itated transducers (IDTs)with re?ectors are fabricated to excite and re?ect the surface acoustic waves (SAWs),and the corresponding oscillator circuits including phase shift circuit,power supply cir-cuit and matching network which are used to maintain the starting condition for oscillation.
In order to detect low concentration levels of hazard gases,various types of thin ?lms and nanostructured materials,such as TiO 2[16,17],ZnO [18–20],SnO 2[21,22],WO 3[23]and poly-mers [24–26],have been applied in the SAW devices due to their high sensitivity to changes in the different gas/vapor atmospheres.Among them,ZnO and SiO 2are commonly used as sensing materi-als because of their chemical sensitivity,high thermal and chemical stabilities,amenability to doping,non-toxicity and low cost [27,28].ZnO/SiO 2nanocomposites are commonly used in photocatalysis [29],photoluminescence [30]and humidity sensing [31].However,the gas sensing property of ZnO/SiO 2bi-layer thin ?lms has rarely been investigated.The mechanism of using the bi-layer gas sensing nano?lm in a SAW sensing system can be based on changes in con-ductivity of a bi-layer thin sensor structure [32].In the case of a single layer ?lm sensing system,it has been addressed previously
https://www.doczj.com/doc/6f7412771.html,/10.1016/j.snb.2014.04.046
0925-4005/?2014Elsevier B.V.All rights reserved.
Y.-L.Tang et al./Sensors and Actuators B201(2014)114–121
115
Fig.1.Schematic diagram of a ZnO/SiO2bi-layer nano?lm based SAW device. by various authors[14,32,33]the relationship between the change of SAW velocity( )versus the sheet conductivity( s)of the?lm can be described as,
0≈?
K2
2
2s
2s+ 2
C2s
(1)
where C s is the surface capacity, s= h is the sheet conductivity, K is an electromechanical coef?cient, 0is the unperturbed SAW velocity.The plot of Eq.(1)is shown in Fig.9.As is shown,if the sheet conductivity( s)is too low or high, changes little despite a huge change in s.The sensor,then,will have no signi?cant response when exposed to gas.But for a bi-layer?lm sensing system,the conductivity of the?lm may fall between those of the two layers which compose the bi-layer?lm.When both layers are very thin in comparison to SAW wavelength and Debye screening length[33], the sheet conductivity of the bi-layer?lm can be adjusted to an appropriate value by changing the thickness of two layers.Thus, will have a relatively great change despite little change in s.As a result,the sensor shows a high response.Moreover,the ZnO and SiO2layers have high and low sheet conductivities,respectively. The bi-layer?lms composed of these two layers,then,may have appropriate sheet conductivity by adjusting the thickness of the two layers.Based on this principle,in this paper,we proposed to use the ZnO/SiO2bi-layer nano?lms with different thicknesses of top ZnO layer on a standard quartz SAW device in order to form a highly sensitive ammonia gas sensor.
2.Experimental details
The SAW resonator was fabricated on a ST-cut(42?75 )quartz substrate(12mm×3mm×0.5mm)with the SAW propagation direction perpendicular to the crystallographic x-axis(90?-rotated) and an acoustic velocity of3158m/s.The input and output alu-minum(200nm)IDTs consisted of30pairs of?ngers with each ?nger width of4?m and a wavelength of16?m.The aperture of the IDTs was3mm.The re?ection gratings had the same geometry with the IDTs,and the SAW resonators had a center frequency of ～200MHz.
Fig.1shows the schematic diagram of the sensor structure. ZnO and SiO2thin layers on top of the SAW device were pre-pared by combining a sol–gel process and a spin-coating technique. Zinc acetate dehydrate(Zn(CH3COO)2·2H2O)was dissolved into a 2-methoxyethanol–monoethanolamine(MEA)solution.The molar ratio of the MEA to zinc acetate was maintained at1.0,and the con-centration of the zinc acetate was0.3mol/L.The resultant solution was stirred at60?C for1h to yield a clear and homogeneous solu-tion,which was served as the coating solution after aged for24h. The St?ber method[34]was adopted for the preparation of silica sol,as explained below.The ethanol(analytic pure),TEOS(high pure),deionized water and ammonia(analytic pure liquid,25wt%) were successively added into a bunsen?ask with a molar ratio of 1:3.25:37:0.17.The resultant solution was stirred at30?C for2h and aged for7h.The concentration of the obtained silica sol was 0.5mol/L.For the preparation of bi-layer nano?lms,the silica sol was?rstly spin-coated onto the SAW devices using a spin-coating process with a speed of5000rpm for30s.Then,the ZnO thin layers with different thicknesses from20to60nm were also prepared
by Fig.2.Picture of the experimental set up for the measurement of frequency shift of SAW device.
a multi-spin-coating process(once for20nm,twice for40nm and three times for60nm).The coated SAW devices were immediately put into a furnace at300?C for about10min,followed by anneal-ing at a temperature of500?C for1h.A?eld-emission scanning electron microscopy(Carl Zeiss1530VP SEM)and an atomic force microscopy(Being technology5500)were used to characterize the morphology of the prepared?lms.
The coated SAW resonator was connected to corresponding oscillator circuits to compose a SAW sensor.The output signal of the SAW sensor was measured using a frequency counter(Agilent 53210).The measurement system is shown in Fig.2.The sensor was mounted inside a testing chamber which had a volume of 1000ml.The standard gas was obtained from National Institute of Testing Technology of China.Ammonia gas was mixed with air and its concentration was5000ppm.A dynamic volumetric method was adopted to conduct the gas sensing experiment.During the gas sensing test,a syringe was used to inject ammonia gas into the testing chamber.Ammonia gas sensing characteristics of the SAW sensor were tested for various ammonia concentrations at room temperature.The concentration(5,10,30,60,90and120ppm)of the ammonia gas was controlled by adjusting the injecting volume (1,2,6,12,18and24ml).
3.Results and discussion
3.1.Structural characterization
Fig.3(a)and(b)are the cross-sectional and top-view SEM images of ZnO and SiO2single layer?lms,with thicknesses of～60 and～70nm,respectively.The ZnO single layer?lm was smooth and dense,whereas the SiO2single layer?lm exhibited many pores and uneven particle sizes.Fig.3(c)shows the SEM images of the ZnO/SiO2bi-layer nano?lm,with a thickness of～130nm.There was no clear boundary between the bottom SiO2layer and the top ZnO layer.The surface morphology of the bi-layer nano?lm was much rougher than that of the single layer ZnO nano?lm,due to the rough nature of the sub-layer of SiO2.There also existed many big particles and many pores on the?lm surface.As a result,the top ZnO layer of the bi-layer nano?lm was porous.
Fig.4(a)–(c)are the AFM images of ZnO single layer nano?lm, SiO2single layer nano?lm and the ZnO/SiO2bi-layer nano?lm with ～60nm ZnO layer.With a surface roughness(RMS)of1.22nm,the ZnO single layer nano?lm was smooth,whereas the RMS value of the SiO2single layer nano?lm was3.61nm due to the existence of pores and uneven particles.The RMS value of the bi-layer nano?lm was4.13nm which was much larger than that of ZnO single layer nano?lm due to the big particles and porous structure.
Fig.5shows the XRD pattern of the bi-layer?lm with～60nm ZnO layer.The peaks at(100),(002),(101)could be obviously
116
Y.-L.Tang et al./Sensors and Actuators B 201(2014)
114–121
Fig.3.Cross-sectional and top-view SEM images of the ZnO single layer nano?lm (a);the SiO 2single layer nano?lm (b);and the bi-layer nano?lm with 60nm ZnO layer (c).
observed.Besides,the peak at (002)had the highest relative inten-sity,which indicated that the top ZnO thin layer had a preferred oriented orientation along (002)direction.Whereas,the bottom SiO 2layer was amorphous.
A four probe method was adopted to measure the conductivity of thin ?lms by using a 4point probes and a digital source meter (Keithley 2400).Table 1shows the measured thickness and sheet conductivities of the ZnO,SiO 2single layer nano?lms and bi-layer nano?lms with 20,40and 60nm ZnO layers.The calculated acous-toelectric parameters ( = s / 0C s )of bi-layer nano?lms just fell between those of single layers ?lms.
Table 1
The measured thickness and sheet conductivity of single layer and bi-layer ?lms.
Single or bi-layer ?lms
Thickness h (nm)
Conductivity s (S/m)
Acoustoelectric parameters
ZnO single layer ?lm 60 1.5×10?695
SiO 2single layer ?lm
707.8×10?110.005Bi-layer ?lm with 20nm ZnO 908×10?100.051Bi-layer ?lm with 40nm ZnO 110 3.2×10?90.2Bi-layer ?lm with 60nm ZnO
130
4.7×10?9
0.3
3.2.Ammonia gas sensing results and discussion
The response characteristics of the SAW sensors with single layer nano?lms to 30and 90ppm ammonia gas with an exposure time of 80s are shown Fig.6,based on the measured frequency shift.The SAW sensor with the SiO 2single nano?lm exhibited a fast response and recovery.The response times of the SiO 2single ?lm coated SAW sensor were ～10and ～15s before reaching 90%of its saturation frequency from the base line,with ammonia gas concen-trations of 30and 90ppm.After saturation,frequency shifts were found decreasing slightly.At the end of tests,frequency shifts were about 100and 175Hz for conditions of 30and 90ppm ammonia gas,and the 90%recovery times were both 3s.On the contrary,the SAW sensor with the ZnO single layer ?lm showed a relatively slow response and recovery time.Frequency shifts increased steadily,and saturation frequency had never been reached.At the end of exposure time,frequency shifts were about 220and 420Hz.The 90%recovery times were 38and 88s.
The responses to the ammonia gas of 30and 90ppm using the SAW devices with bi-layer nano?lms with 20,40and 60nm top ZnO layer are shown in Fig.7.It can be seen that the absolute response increased signi?cantly,but the response and recovery times also increased with the increase of the thickness of top ZnO
Y.-L.Tang et al./Sensors and Actuators B 201(2014)114–121
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Fig.4.AFM image of the ZnO single layer nano?lm (a);the SiO 2single layer nano?lm (b);and the bi-layer nano?lm with ～60nm ZnO layer (c).
layer.Table 2lists the frequency shifts and response and recovery time for the above here devices of different top ZnO layer thick-ness.From Table 2,clearly,the SAW device with 60nm top ZnO layer showed the best performance.The explanation will be given in the following
part.
Fig.5.XRD pattern of the bi-layer nano?lm with ～60nm ZnO layer.
Fig.8shows the response times,recovery times and absolute responses of ?ve nano?lms,including ZnO,SiO 2single layer and the bi-layer nano?lms.As is shown in Fig.8(a)and (b),the response and recovery times of the SAW devices with the bi-layer nano?lms were
Table 2
The sensing results of the bi-layer ?lms with 20,40and 60nm ZnO layer.
Sensors
Ammonia gas concentration (ppm)
Absolute
response (Hz)
90%response time (s)
90%
recovery time (s)
The sensor with 20nm ZnO
30
300
22
5
90800135The sensor with 40nm ZnO
30
500
30
10
9015003034The sensor with 60nm ZnO
30
2000
38
30
90
4500
32
54
118Y.-L.Tang et al./Sensors and Actuators B201(2014)114–121
Fig.6.Frequency responses of SAW sensors with single layer nano?lms to30ppm(a)and90ppm(b)ammonia gas.
Fig.7.Frequency responses of SAW sensors with bi-layer nano?lms to30ppm(a)and90ppm(b)ammonia gas.
Fig.8.Response times(a),recovery times(b)and absolute responses(c)of?ve nano?lms.(The“sl”and“bl”in brackets indicate the single layer and bi-layer nano?lm, respectively).
Y.-L.Tang et al./Sensors and Actuators B201(2014)114–121
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Fig.9.Plot of velocity changes versus acoustoelectric parameter = s/ 0C s.(The “WP”in brackets indicates working point.)
between those with the single SiO2and ZnO layers.The top ZnO layer of the bi-layer?lm had an effect of increasing the response and recovery time.This can be attributed to its porous structure. With the increase of the thickness of the ZnO thin layer,the den-sity and length of the pores increased.As a consequence,it would take much more time for ammonia gas to diffuse in and go through the ZnO thin layer.Fig.8(c)shows the absolute responses of the?ve types of the nano?lms.It can be observed that the SAW devices with the bi-layer nano?lms showed a much higher sensitivity.With the increase of the thickness of top ZnO layer for the bi-layer?lm struc-ture,the sensitivity of the SAW device enhanced signi?cantly.This can be attributed to changes in conductivity of a bi-layer thin sen-sor structure[32].As mentioned in Section3.1,the acoustoelectric parameters of the ZnO single layer?lm,the SiO2layer?lm and the bi-layer?lms were95,0.005,0.051,0.2and0.3.The plotted curve of Eq.(1)for these single layer and bi-layer structures are presented in Fig.9,where the acoustoelectric parameter = s/ 0C s,h=60, 70,90,110and130nm,and =16?m(the curves of h=60,70,90, 110and130nm are almost the same).The?ve different working points of the different?lms are marked in Fig.9as well.The value of absolute response(frequency shift, f)of sensors versus velocity change( )can be described as f= / .Thus,the sensitivity of the?ve?lms is dependent on the slope of the curve where the working points located.It can be observed that when used as sin-gle layer?lms in the SAW device individually,the working points of the ZnO and SiO2single layer?lms located in regions of low sensi-tivity,where the slopes of the curve were about0,due to their low and high acoustoelectric parameters.Within these regions,even though there were great changes in the sheet conductivities,the two?lms would show very weak responses to ammonia gas.Thus, the two single layer?lms showed very poor gas sensing property.On the other hand,for the bi-layer nano?lm with60nm top ZnO layer,which was with an appropriate acoustoelectric parameter, its working point was in a region of high sensitivity.Thus,when exposed to ammonia gas,the changes in the sheet conductivity of this?lm would cause a great shift in SAW velocity.That resulted in the higher sensitivity of this bi-layer nano?lm.Moreover,the bi-layer?lms with thinner top ZnO layers had smaller sheet con-ductivities which resulted in smaller slopes.As a consequence,the sensitivity of these?lms were,then,worse than that of the?lm with60nm top ZnO layer.
The ammonium sensing mechanism using the bi-layer SAW sensor can be described as follows.The oxygen molecules?rstly absorbed to the surface of the top ZnO layer due to the van der Waals force.Then,because of their strong oxidizability,the absorbed oxy-gen molecules can get the super?cial free electrons,leading to a depletion layer.When the?lm was exposed to ammonia gas,some ammonia molecules absorbed to the surface of the top ZnO layer. These molecules would diffuse and even penetrated in the?lm due to its porous structure.Then,the ammonia molecules reacted with the oxygen species causing lots of electrons to return to the deple-tion layer;these electrons would recombined with some of the holes,which resulted in a change in the conductivity of top ZnO layer.When these ammonia molecules unceasingly diffused into the porous bottom SiO2layer,the sheet conductivity of bottom SiO2 layer changed as well.The ammonia gas sensing mechanism is as follows[35],
2NH3+5O?(abs)→2NO+3H2O+5e?(2) Thus,the conductivities of the two layers increased due to the more free electrons.As a consequence,the sheet conductivity of the bi-layer nano?lm composed of these two layers increased as well. Then,the value of the acoustoelectric parameter increased,which caused a negative shift in SAW velocity(the red arrow in Fig.9).As a result,a negative shift in frequency would be observed.
Response to varying concentrations of ammonia gas for the SAW device with a bi-layer nano?lm with60nm of top ZnO layer is shown in Fig.10(a)and the sensing results are listed in Table3. Results show that the sensing response was much smaller but the recovery was faster at a lower concentration.Some discon-tinuous points were observed during the recovery process.These points were caused by the mechanical disturbance when evacuat-ing ammonia gas.
The reproducibility of the sensor based on the bi-layer nano?lm with60nm top ZnO layer were further tested by exposing the devices with30ppm ammonia gas for5cycles.As is shown in Fig.10(b),the?uctuation of frequency shift was less than10%,
and Fig.10.Frequency response of SAW sensors based on bi-layer nano?lm with(a)60nm ZnO coating to ammonia gas at different concentrations;(b)30ppm ammonia gas for?ve cycles.
120Y.-L.Tang et al./Sensors and Actuators B201(2014)114–121
Table3
The sensing results of the bi-layer?lm with60nm ZnO.
Ammonia gas concentration (ppm)Absolute
response(Hz)
90%response
time(s)
90%recovery
time(s)
55006516
1010005018
3020004028
6035003836
9045003452 12058003056
the response and recovery times was similar for the?ve consecutive tests,revealing a good reproducibility and stability.
4.Conclusions
Ammonia gas sensors based on the quartz SAW devices using bi-layer nano?lms of ZnO and SiO2were investigated.The top ZnO and bottom SiO2layers were fabricated using sol–gel and spin-coating methods.The bi-layer?lms had porous structures which were ben-e?cial for the ammonia gas to diffuse in and pass through.The value of acoustoelectric parameters of sensing?lms was found to have a signi?cant in?uence on the sensitivity of the sensors.The ZnO and SiO2single layer?lms had very poor sensitivity due to their excessively high and low acoustoelectric parameters,whereas,the bi-layer?lms showed the appropriate acoustoelectric parameters, which resulted in the good sensitivity at room temperature.The bi-layer?lms also showed a good reproducibility and stability.The absolute response value,the times of response and recovery of the bi-layer nano?lm was dependent on the thickness of top ZnO layer. With the increase of the thickness of the top ZnO layer,the abso-lute response value of bi-layer nano?lm enhanced rapidly,while the response and recover times increased.
Acknowledgement
This work was supported by the Fundamental Research Funds for the central Universities(ZYGX2012J047),the Joint Fund of the National Natural Science Foundation of China and the China Academy of Engineering Physics(U1330108)and the National Nat-ural Science Foundation of China(No.11304032).
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Biographies
Yong-Liang Tang obtained his B.S.degree in School of Physical Electronics,University of Electronic Science and Technology of China in2007.He is a Ph.D.student in School of Physical Electronics at University of Electronic Science and Technology of China. His present interests include applications of nanomaterials and functional thin?lms for sensors and surface acoustic wave(SAW)devices.
Zhi-Jie Li received his Ph.D.degree from Institute of Coal Chemistry,Chinese Academy of Sciences in2005.He is an Associate Professor in School of Physical Electronics at University of Electronic Science and Technology of China.His current interests are nanomaterials,novel functional materials and their spectroscopy.
Jin-Yi Ma received his Ph.D.degree from Tianjin University in2003.He is a senior research fellow in Sichuan Institute of Piezoelectric and Acousto-optic Technology. His research area is on piezoelectric materials and industrial applications of the acoustic-optical device.
Yuan-Jun Guo received his Ph.D.from Shanghai Institute of Optics and Fine Mechan-ics,Chinese Academy of Sciences in2006.From July2011,he became an Associate Professor in School of Physical Electronics,University of Electronic Science and Technology of China.His current research interests focus on sensing devices using acoustic wave technology,micro?uidics and interaction between laser and solid material.
Y.-L.Tang et al./Sensors and Actuators B201(2014)114–121121
Richard Yong-Qing Fu received his Ph.D.degree from Nanyang Technological Uni-versity,Singapore in1999,and then worked in University of Cambridge and Heriot-Watt University,UK.He is a Reader in Thin Film Centre in University of the West of Scotland.His recent research has been focusing on microactuators, microsensors and micro?uidic devices based on smart functional thin?lms.Xiao-Tao Zu received his Ph.D.degree from Sichuan University in2002.He is a Professor in School of Physical Electronics at University of Electronic Science and Technology of China.His research interests include photoelec-tric materials,smart materials,composite nanomaterials and their industrial applications.